Just as a practical matter
Imagine a President in the White House with a Congressional party of zero.
Sure, anyone can run. Theoretically, anyone can win. But in order to govern, one must have some support in the legislative branch.
Ralph Nader's political power is negative. If elected he would have no real power to govern. He only has the power to take away. He has no power to give. Unless you count the sitting President as his recipient.
